The heating of slabs or objects in such furnaces is not only achieved directly by means of roof burners and/or by the radiation of heat from the roof but also by means of radiation reflected from the side walls. It has been established that rolled strips from slabs which had been heated in such a furnace having vertical side walls have a different quality at the leading edge and the end of the strip as compared to that in the rest of the area of the strip. Such differences in quality are particularly undesired for transformer sheet steel, for example, experiments have shown that peak temperatures in the area of the ends of the slabs are responsible for this different quality of the strip over its length. This different heating is due to the radiation from the roof, which is reflected on the side walls of the furnace and which strikes the ends of the slabs, being most intensive thereat while its intensity decreases towards the middle of the slab such that it becomes negligibly small thereat in comparison to the direct radiation prevailing there.
In order to prevent this disproportionate heating of the slabs, attempts have been made to provide niches in the side walls in the area of the conveying paths of the ends of the slabs, which niches protect the ends of the slabs against the radiation heat of the furnace walls. Such a furnace has certain structural difficulties due to the presence of the niches.